Retro-fitting irrigation system increases efficiency

With cost-share funds available for some Georgia farmers to upgrade their center pivot irrigation systems, now might be the time to consider a more efficient method of watering your crops, says Kerry Harrison, University of Georgia Extension engineer.

“There are cost-share programs available for retro-fitting center pivots,” says Harrison. “Your cost share will be available through your local Soil and Water Conservation Commission or NRCS office. Depending on where you’re located in Georgia, funds are available for cost-share to help change out worn sprinklers on center pivot systems. This program will tell you if it’s time to change sprinklers — it’s all based on uniformity.”

Several sprinkler packages currently are available to producers, he says. “It’s not a cookie-cutter approach — it’s not A or B. There are several choices. You may not have a choice in some situations, depending on cost-share stipulations.

“There are a lot of considerations when selecting which type of sprinkler or irrigation system you need, and how you’ll manage that system. The crop being watered also is important. Corn, for example, responds very well to irrigation,” says Harrison.

There also are physical limitations, he adds, including the size and shape of the field, the water source, and your management limits and capabilities. Each grower has a different system and a different management style, says Harrison, and what works for your neighbor may or may not work for you.

Irrigation came to Georgia many years ago, he says, and most of the center pivot systems were equipped with 23-degree angle impact sprinklers. This sprinkler mounts on top of the pivot and shoots water up at a 23-degree angle.

“That’s old technology, and the best thing I can say about it is if you still have those sprinklers, get rid of them. You could be paid cost-share to replace those sprinklers. Basically, we’re talking about a sprinkler that is 15 to 20 years old or older. But we still have some of these systems out there, and they have their original sprinklers.”

The more you spray water into the air, the more you waste and lose, says Harrison.

“The buzz word now is ‘efficiency.’ And by efficiency, we’re talking about what percentage of the water actually gets to the soil and is subject to being taken up by the plant and being used. The more we spray water into the air, the more chance we have for a loss.”

Efficiency isn’t the same as uniformity, he says. “Any system can be uniform. But if we want to be efficient, we need to try and get as much water to the crop as possible. Those old sprinklers, although good in their day, have proven not to be the most effective method of getting water to the crop.

“The only thing worse — efficiency wise — would be a traveler. Some farmers still are limited to using a traveler, but it’s not the most efficient delivery method.”

The next step, says Harrison, would be a low-angle impact sprinkler. “We take that 23-degree trajectory, and we change it to a 6-degree package. At this point, we’re in the low-pressure category. On some scale, a low-angle impact is considered low pressure. It is an option, and it should be considered on an individual basis.”

One step further, he says, would be a spray nozzle located on top of the pivot. This is a group of umbrella-type patterns that continually spray water.

“With this type system, we’re changing how much time water is exposed to the air and the wind, for evaporation purposes. We’re making the delivery more efficient, maybe 3 to 5 percent more efficient.”

The next step, he continues, would be to take that nozzle, and instead of putting it on top of the pivot, put it on a drop that stops at about truss-rod height or the tassel of the corn. This system has advantages and disadvantages, but you’re never going to have a perfect situation, 100 percent of the time, he says.

“Farmers don’t grow the same crop year after year. Rather, they have a rotation. If you use a drop sprinkler package, and you grow a variety of corn that grows up into that sprinkler, it’s going to disrupt the pattern and affect the uniformity of delivery. You have to decide if that’s unacceptable for your situation.

“This may occur only one year out of every three or four in your rotation. And the corn will be at that stage for only two or three weeks. It’s not perfect, and you’ll have problems with canopy interference during some part of the production cycle, but that’s a management decision.”

There are spray nozzles, says Harrison, with fixed and rotational patterns. “We haven’t really researched the efficiency of these two methods, but I doubt that there would be a statistical, measurable difference between those two types of applications.”

It has been proven that a pattern that moves will have better uniformity than a stationary one, he says.

“Again, we’re only talking about a small, 1 to 2 percent advantage in uniformity. The choice of which particular brand-name or type of sprinkler would be between you and your irrigation dealer. Name-brands haven’t been researched very thoroughly, and if they were, I don’t think we’d see any statistical differences.”

If you take a drop spray nozzle that is at truss-rod height and move it down to about knee-high, you would have LEPA — low-energy pressure application, he says. This is the most efficient delivery method available. Water is not exposed to the air for very long, and since it’s low pressure, it’s more energy efficient.

“But we have one big problem with LEPA. Because it is at such a low elevation, we have canopy interference. How do we make it work in tall-growing crops? There’s only one way, and that’s to plant the field in a circle, and not too many farmers are willing to do that.

“In Georgia, we prefer to plant fence-row to fence-row. Very few people will plant in a circle because you have to keep that drop in between the two rows. They’ll break cotton or corn plants.”

The second problem. he says, is that the nozzle is only about 18 inches off the ground. “With our field elevations, the spray nozzle will start to drag even in peanuts. Our elevation differences in south Georgia are too great for LEPA.”

Research has shown that the efficiency difference between LEPA and other systems is about 6 to 8 percent in normal situations, says Harrison.

The major problem with any form of sprinkler change — especially low pressure — is the over-application of water or excessive application rates, he says. It takes a certain flow rate to put out 1 inch of water in a certain amount of time. Regardless of the pressure at which the water is being delivered, you have to deliver the flow rate, he adds.

“For instance, if I have an impact system at 800 gallons per minute, and I change it to a spray nozzle or drop, it still has to be 800 gallons per minute. Low pressure doesn’t take less water. Low pressure takes less energy. We still need the same flow rate to get the water out there.

The maximum application rate is out at the last tower or at the end of the center pivot, he says. “When I’m asked to calculate this, I look in the NRCS handbook for the soil type, and the soil type tells me it has an uptake capacity of a certain number of inches per hour. “For Tifton sandy loam with conservation-tillage, the uptake rate is 3.25 inches per hour. I plug that into my formula. I know how long the system is, the wetted radius of the pivot, and how many gallons per minute the pivot is nozzled for. I solve for the radius of the sprinkler. How far do I need to throw that water at the last sprinkler to maintain an application rate that the soil can handle?“

In this example, the radius of the last sprinkler needs to be 42 feet. “I don’t need exactly 42 feet — I’ll take 36 or 45. What sprinkler do I need for that pivot to have that kind of radius? A spray nozzle won’t do it — it’s radius is 25 feet at best. For these numbers, the pivot needs an impact sprinkler, or it will apply water at a rate faster than the soil can take it up.”

This formula isn’t a bible, says Harrison. It’s an indicator to get you in the ballpark.

“With the cost-share money, one program requires that you convert to sprays-on-drops. If this indicator says this is the wrong sprinkler package, then you don’t need it. Everything doesn’t need spray-zone drops. Different sprinkler packages will work in different conditions. The closer to the ground, the more efficient the delivery.”

All farmers, says Harrison, need to be good stewards and demonstrate that they are doing the right to be more efficient in their water use.

“Cost-share funds are available for changing to end-gun shut-offs that work better than the one you have or don’t have. We have add-on units that have a GPS locator that’ll turn the end guns on or off, and you can add it these to any pivot without doing a pivot panel upgrade. The cost is about $800.”